Device for positioning and axially aligning a fuel assembly and process and apparatus for restoring a positioning element

ABSTRACT

The apparatus for positioning a fuel assembly ( 10 ) comprises, on a respective side of the core support plate ( 19 ) and the upper core plate ( 9 ) directed towards an endpiece ( 15, 14 ) of the fuel assembly ( 10 ) at least two projecting positioning pins ( 16, 22 ) adapted to engage in openings in the lower endpiece ( 15 ) or upper endpiece ( 14 ). Each of the positioning pins ( 16, 22 ) is fixed in a blind opening ( 24 ) comprising a threaded bore and a support surface perpendicular to the axis of the blind opening ( 24 ) and comprises a flange adapted to come into contact with the support surface when the positioning pin ( 16, 22 ) is screwed into the threaded bore. A gudgeon enables the positioning pin to be locked to secure it against unscrewing. The invention also relates to a process and apparatus for replacing damaged positioning pins on the core support plate ( 19 ) or on the upper core plate ( 9 ).

This application is a divisional of U.S. patent application Ser. No.10/458,732, filed Jun. 11, 2003, now U.S. Pat. No. 6,888,912.

FIELD OF THE INVENTION

The invention relates to an element for positioning and axially aligninga fuel assembly in the core of a water cooled nuclear reactor andparticularly a pressurised water-cooled nuclear reactor. The inventionalso relates to a process and apparatus for restoring a positioningelement of the fuel assembly.

BACKGROUND OF THE INVENTION

Pressurised water cooled nuclear reactors comprise inside a tank a coreconsisting of fuel assemblies of generally straight prismatic shapeplaced in juxtaposed arrangements with their axes running vertically.

The nuclear reactor comprises a support plate for the core of thenuclear reactor placed in a horizontal position inside the tank of thereactor, on which the fuel assemblies rest via a lower endpiece.

The core support plate constitutes some of the lower internal equipmentof the nuclear reactor which comprises, above the core support plate,partitioning for holding the peripheral assemblies of the core and acylindrical core envelope arranged in the tank in a coaxial position.

After the core of the nuclear reactor has been loaded by successivelyplacing each of the core assemblies on the support plate inside thepartitioning, upper internal equipment of the reactor is put intoposition above the core, this equipment comprising in particular anupper core plate bearing on the upper parts of the fuel assembliesconstituting the upper endpieces of these assemblies.

The core assemblies are held inside the tank, in their operatingposition, by support or abutment means and positioning means for theupper internal equipment and lower internal equipment.

The fuel assemblies of the core must be kept in perfectly definedpositions inside the operating nuclear reactor so that the cooling waterof the reactor flowing vertically from bottom to top in contact with theassemblies of the core, after having passed through the lower supportplate of the core through water accommodation openings is able toeffectively cool all the fuel rods held in the frameworks of the fuelassemblies in the form of bundles of parallel rods.

The fuel assemblies are held perfectly aligned in juxtaposed positionsby positioning elements at the level of the lower core plate and thelevel of the upper core plate. In particular, the core support plate andthe upper core plate comprise positioning elements having positioningpins adapted to cooperate with the openings provided, respectively,through the lower endpieces and upper endpieces of the fuel assemblies.

The positioning elements generally consist of pins having means forfixing in the support plate or in the upper core plate, inside openingsextending axially perpendicular to the plate, so that the positioningpins project either upwards in the case of the core support plate ordownwards in the case of the upper core plate.

The lower endpiece of the fuel assemblies, which is generally square,has two openings arranged along a diagonal of the endpiece which areadapted to engage on two positioning studs projecting relative to thecore support plate at locations which ensure adequate positioning of thefuel assembly.

When the core has been fully loaded, the fuel assemblies resting on thecore support plate are positioned by means of the pins on which theirlower endpieces engage in well defined relative positions which ensureperfect juxtaposition of the fuel assemblies in the core. After loadingof the assemblies of the core the upper internal equipment whichcomprises a lower plate constituting the upper core plate is put back inposition. This upper core plate comprises, as mentioned above,positioning pins which engage in openings in the upper endpieces of thefuel assemblies. The fuel assemblies are thus positioned and held attheir bottom end and top end by the lower and upper core plates,respectively. This ensures perfect juxtaposition and axial alignment ofthe fuel assemblies inside the core. Also, by the fact that the upperinternal equipment bears on the upper endpieces, the fuel assemblies areheld counter to any lifting effect by the cooling water of the nuclearreactor circulating upwards in contact with the fuel assemblies of thecore. The positioning studs for the elements for positioning and holdingthe fuel assemblies make it possible in particular to ensure that thereis substantially constant play between the fuel assemblies along theirentire periphery and over the entire surface of the core. This ensuresregular distribution of the cooling water throughout the core of thenuclear reactor.

The positioning pins ensure on the one hand that the fuel assemblies arecentered while the core is being loaded in the lower internal equipmentand then centered and held relative to the upper internal equipment whenthey are put into position above the core. Moreover, the positioningpins ensure that the forces produced by the circulation of the coolingwater during normal operation of the reactor are absorbed as mentionedabove, as well as the forces produced in the event of an accident, forexample of APRP type (Accident through loss of primary coolant) orduring seismic activity. The positioning and maintenance of the perfectpositioning of the fuel assemblies in the core therefore require thepresence of all the positioning pins of the lower support plate of thecore and of the upper core plate and perfect alignment of thepositioning pins in the axial direction to ensure that the fuelassemblies are aligned. The positioning pins of the fuel assemblies arefixed in respective fixing openings in the core support plate and uppercore plate which are produced so as to give perfect alignment of thepositioning pins and hence perfect alignment of the fuel assemblies.

Generally, the fixing openings for the positioning pins in the uppercore plate are through-openings in which part of the positioning pinengages, this part comprising in particular a threaded end onto which anut is screwed from the upper part of the upper core plate in order tosecure and lock the pin inside the fixing opening passing through theupper core plate. The centering and locking of the positioning pininside the opening passing through the upper core plate may be carriedout in different ways by providing contact surfaces for the pin and thefixing opening which are adapted to cooperate in order to ensure perfectalignment and centering.

The positioning pins are generally fixed to the core support plateinside blind openings comprising a threaded bore into which is screwed athreaded part of the positioning pin. The positioning pin is generallyprevented from rotating by means of a washer which is itself securedagainst rotation and fixed by spot welds in a counter bore of the fixingopening for the core support plate.

The positioning pins of the fuel assemblies which comprise a partprojecting below the upper core plate or above the core support platemay be damaged during handling of the fuel assemblies or the internalequipment of the reactor. This damage may take the form of deformation,for example bending, of the projecting part of the positioning pin andin some case may even amount to total breakage of the projecting part ofthe positioning pin.

If certain positioning pins are deformed and are no longer in perfectaxial alignment it may be difficult to position the fuel assemblies inthe core and/or the upper internal equipment above the core, whichpresupposes perfect engagement of the positioning pins projecting fromthe core support plate and below the upper core plate, into the openingsin the lower or upper endpieces of the fuel assemblies. If thepositioning of the fuel assemblies and upper internal equipment can becarried out, deformation of some positioning pins may lead to defectivepositioning of the fuel assemblies. The same is true if one or morepositioning pins are missing from underneath the upper core plate. Inthis case, some of the fuel assemblies of the core might be positionedor held in defective positions inside the core. There may then be, forexample, a considerable reduction in the play between adjacent fuelassemblies, resulting in an increase in the loss of charge in thecirculation of the cooling water through the reactor. This loss ofcharge may be substantial enough to cause blockage of the watercirculation along the fuel assemblies in at least one zone of the core.Then, in this poorly cooled zone of the core, overheating may occurwhich may result in the fusion of certain elements or fuel rods in thecore assemblies.

In the case of positioning pins projecting relative to the upper coreplate, at the lower end of the upper internal equipment, processes andapparatus for replacing pins comprising positioning studs which aredefective or absent have been proposed. Generally, the replacementprocesses or apparatus require intervention above the upper core plateto gain access to the screws for fixing and securing the positioningpins. Such intervention from the top of the upper core plate requiresdismantling and replacement of certain parts of the upper internalequipment, which means that the time taken to replace the positioningpins is extended and therefore the cost of the operation is increased.

EP-0.718.851 proposes an apparatus for aligning fuel assembliescomprising a positioning pin which can be removed and replaced entirelyfrom below the upper core plate and upper internal equipment. However,the apparatus proposed is complex and requires the use of ascrew-threaded locking element of the replacement pin.

In certain cases it has also been found that one or more positioningstuds on the core support plate has been absent or deformed.

The positioning studs for the fuel assemblies on the support plate ofthe core generally consist of an end portion of a positioning pin whichcomprises, successively, in its axial direction, the positioning studand a portion which is at least partially threaded adapted to beinserted and screwed into a screw thread in the opening of the coresupport plate. The positioning element for the fuel assembly comprises,in addition to the positioning pin, a washer for securing the pinagainst rotation, which comprises an opening having two straight edgesin which part of the pin with two flat counter bores engages. The washeris inserted in a counter bore machined on the upper surface of the coresupport plate around the fixing opening for the pin, and fixed by spotwelds.

When a fuel assembly positioning element is to be re-built as a resultof the positioning pin being missing, damaged or broken, so that thepositioning stud cannot be used any longer or is missing entirely, thework to restore the positioning element has to be done under water; infact, the internal equipment of the nuclear reactor is irradiated aftera period of operation of the nuclear reactor.

During a phase when the nuclear reactor has been shut down and cooled,the core of the reactor is reloaded and any repairs needed to theinternal equipment of the tank are carried out.

After the lid of the tank has been opened under water, the upperinternal equipment is dismantled and placed on an inspection andchecking stand in the pool of the nuclear reactor, then the coreassemblies are removed, which are generally emptied into a pool for thefuel.

It is then possible to gain access to the lower internal equipment,either in order to intervene directly on this lower internal equipmentinside the tank of the reactor, or to transport it onto an inspectionand repair stand in the pool of the reactor.

It is also possible to intervene on the upper internal equipment placedon the inspection and checking stand, for example in order to replacepositioning pins on this upper internal equipment. It is thereforeextremely important that the positioning pins for the apparatus forpositioning and axially aligning the fuel assemblies are produced so asto assist their assembly on the new internal equipment in the factoryand their replacement on worn internal equipment during a restoringoperation carried out under water in a pool.

The aim of the invention is therefore to propose an element forpositioning and axially aligning a fuel assembly in the core of a watercooled nuclear reactor, consisting of fuel assemblies of straightprismatic shape each resting, via a lower endpiece, on a horizontalsupport plate for the core, with its axis extending vertically, and heldby an upper core plate resting on an upper endpiece of the fuelassembly, comprising, on a respective surface of the core support plateand upper core plate directed, respectively, towards the upper endpieceand the lower endpiece of the fuel assembly, at least two projectingpositioning pins each fixed in an axial direction of alignment of thefuel assembly, inside a fixing opening for pins, in order to engagerespectively in positioning openings in the lower endpiece and upperendpiece of the fuel assembly, this apparatus also assisting the initialmounting in the factory as well as any eventual replacement, on wornelements in the nuclear reactor, of positioning pins of fuel assemblies.

To this end:

-   -   each of the fixing openings for pins in the core support plate        and upper core plate comprises a blind threaded bore and a        support surface perpendicular to the axial direction of the        fixing opening having at least one dimension greater than a        diameter of the blind threaded bore, extending around the bore,    -   each of the positioning pins has a threaded shaft to enable it        to be secured by screwing in the threaded bore of the fixing        opening and an annular flange having a dimension greater than        the diameter of the shaft for bearing on the support surface,        and    -   each of the positioning pins is retained, after screwing and        locking in the core support plate or in the upper core plate, by        a gudgeon engaging in the flange of the positioning pin and in a        cavity in the corresponding plate and made inseparable from the        positioning pin and the corresponding plate.

The invention also relates to a process for restoring a positioningelement for a fuel assembly on a worn component of the nuclear reactor.

To ensure that the invention is properly understood, the core of apressurised water cooled nuclear reactor, the positioning elements of afuel assembly of the core and an operation for restoring a positioningelement of a fuel assembly on the support plate of the core will now bedescribed by way of example, with reference to the accompanyingdrawings.

FIG. 1 is a section through a vertical plane of a tank of a pressurisedwater nuclear reactor containing the core of the nuclear reactor.

FIG. 2 is an elevation and partial section of a fuel assembly of thecore of the nuclear reactor and a device for positioning and aligningthe fuel assembly in the core of the nuclear reactor.

FIG. 3 is an elevation and partial section through a vertical plane of apositioning pin of a device for positioning a fuel assembly according tothe invention.

FIG. 4 is an enlarged view of detail 4 in FIG. 3.

FIG. 5 is an elevation and section through a vertical plane of anelement for positioning fuel assemblies on the core support plate of anuclear reactor according to the prior art.

FIG. 6 is a cross section along 4—4 in FIG. 3, of the positioningelement.

FIG. 7 is a perspective view of a tool support plate for carrying outthe process according to the invention on the core support plate of apressurised water nuclear reactor.

FIG. 8 is a perspective view of the tool support plate placed on thecore support plate in order to restore a positioning element.

FIG. 9 is a plan view of the tool support plate in position on the coresupport plate.

FIG. 10 is a perspective view of a machining tool in position on thetool support plate for carrying out the process according to theinvention.

FIG. 11 is an enlarged perspective view of the end part of the machiningtool shown in FIG. 10 during the machining step of the process accordingto the invention.

FIG. 12 is a perspective view of an extraction tool in the workingposition on the tool support plate during a stage of extracting thelocking washer from the positioning element during the restoringprocess.

FIG. 13 is an enlarged view of part of the extraction tool in theworking position above the positioning element which is to be rebuilt.

FIG. 14 is an elevation and partial section of a screwing tool for thereplacement bushing used during a final step of the process forrestoring the positioning element.

FIGS. 15A and 15B are a plan view and elevation, respectively, ofcentring means for tools used for carrying out the process according tothe invention.

FIG. 1 shows the tank of a pressurised water nuclear reactor generallydesignated 1 which is closed-off at the top by a removable cover 2.

Inside the tank 1 are the upper internal equipment 3 and lower internalequipment 4 of the nuclear reactor. The lower internal equipment 4comprises in particular the support plate 19 for the core of thereactor, a core envelope 6 suspended in a coaxial arrangement inside thetank 1 and integrally connected at its lower end to the core supportplate 19 as well as partitioning 7 arranged inside the core envelope 6and adapted to contain and hold the fuel assemblies which constitute thecore 8 of the nuclear reactor.

The upper internal equipment 3 comprises in particular the guide tubesfor the bars for controlling the reactivity of the core of the nuclearreactor and the upper core plate 9 by means of which the upper internalequipment 3 is made to bear on the upper parts of the fuel assemblieswhich constitute the core 8 of the nuclear reactor.

The fuel assemblies 10 which constitute the core of the nuclear reactorhave a straight prismatic shape, generally a parallelepiped shape with asquare base the axis of which is vertical in the operating position ofthe assemblies 10 inside the core.

As can be seen from FIG. 2, each of the assemblies 10 of the core of thenuclear reactor consists of a set of fuel rods 11 arranged in the formof a bundle in which the elongated cylindrical rods are placed withtheir axes vertical inside a framework comprising in particular spacergrids 12 for holding the rods 11 in the form of a bundle in which therods are arranged in a regular grid-shaped pattern in transverse planesperpendicular to the axial direction, guide tubes 13 connecting thespacer grids 12 distributed in the axial direction of the fuel assembly10 and two endpieces 14 and 15 fixed to the end portions of the guidetubes 13 and closing off the fuel assembly.

The endpiece 14 provided at the upper end of the fuel assembly, known asthe upper endpiece, comprises in particular openings for receivingpositioning studs 16 fixed to the upper core plate 9 and a set ofsprings 17 on which the upper core plate 9 abuts in order to hold thefuel assembly while allowing longitudinal expansion of the fuel assemblyin accordance with the temperature to which the fuel assembly issubjected.

The lower endpiece 15 of the generally square fuel assembly comprises,in each of its angles, a support foot which abuts on the core supportplate 19 in the operating position of the fuel assembly 10. Two of thefeet of the fuel assembly, arranged along a diagonal of the squareendpiece, comprise openings 21 in each of which engages a stud of apositioning element 20 of the fuel assembly constituting the partprojecting above the core support plate 19 of a positioning pin 22.

FIG. 3 shows an element for positioning the fuel assembly generallydesignated 20. By way of example, a positioning element is shown fixedto the core support plate 19 comprising a positioning stud adapted toengage in an opening in the lower endpiece 15 of the fuel assembly. Thepositioning element of the fuel assembly comprises positioning elementsfixed to the lower core plate 5 and also positioning elements fixedunderneath the upper core plate 9, as shown in FIG. 2.

We will not describe in detail how the positioning element 20 is fixedto the core support plate 19 and shown in FIG. 3, a positioning elementfixed underneath the upper core plate 9 being produced in the same wayas the element 20 shown in FIG. 3 but directed downwards and comprisinga positioning stud projecting relative to the upper core plate, engagingin a positioning opening of the upper endpiece 14 of the fuel assembly.

The positioning element 20 shown in FIG. 3 comprises a positioning pin22 having two main parts, the stud 22 a adapted to project above thecore support plate 19 in the axial direction 23 and a threaded fixingportion 22 b in the axial extension of the stud 22 a adapted to bescrewed into a blind opening 24 in the core plate 19 which is threadedover at least part of its length. The fixing opening 24 of thepositioning element 20 and the positioning pin 22 fixed in a coaxialarrangement inside the opening 24 constitute the entire positioningelement 20.

The positioning element of each of the fuel assemblies comprises twopositioning elements 20 at the core support plate 19 and two positioningelements at the upper core plate 9 which are positioned so that thestuds for positioning and axially aligning these positioning elementsengage, respectively, in two openings in the lower endpiece 15 of thefuel assembly and in two openings in the upper endpiece of the fuelassembly arranged along a diagonal of the endpiece.

The stud 22 a of the positioning pin 22 has two flattened areas 27 usedas gripping surfaces for a screwing tool when the pin is screwed intothe threaded opening 24, either during the initial mounting of the fuelassembly positioning elements on the new internal equipment of thenuclear reactor or during the screwing of a replacement pin in athreaded opening in a worn piece of internal equipment, during anoperation to restore positioning elements, as will be describedhereinafter.

Between the stud 22 a and the threaded portion 22 b, the pin 22comprises an annular flange 30 and optionally, following the flange 30,in the direction of the threaded portion 22 b, a smooth portion 25 whichis smaller in diameter than the threaded portion 22 b.

The external diameter of the flange 30 is substantially equal to orslightly less than the diameter of a counter bore 26 machined in theupper part of the core support plate 19 around the opening 24.

The flange 30 comprises a flat lower surface 30 a of annular shapeadapted to bear against the surface ˜26 a of the base of the counterbore, the contact of one of the surfaces 30 a and 26 a against the otherbeing intended to ensure perfect positioning of the positioning pin andin particular a direction of the axis 23 of the positioning stud 22 awhich is perfectly perpendicular to the upper bearing surface of thecore support plate 19. A perfectly coaxial position between thereplacement pin 22 and the opening 24 is ensured, as will be describedhereinafter, by the precision of positioning the axis of the threadedportion of the opening 24 adapted to accommodate the threaded portion 22b of the positioning pin 22.

As can be seen from FIGS. 3 and 4, the positioning element 20 furthercomprises a gudgeon 31 for securing the positioning pin 22, the gudgeon31 being tubular in shape and engaging in a cavity 32 machined after thepin 22 has been put into position, through the flange 30 and in the coresupport plate 19, the cavity 32 having an axis 32′ substantially at atangent to the outer edge of the flange 30 and to the inner edge of thecircular counter bore 26.

During the manufacture of the internal equipment of a new nuclearreactor in the factory, the fuel assembly positioning elements areproduced by drilling the core support plate of the lower internalequipment and the upper core plate of the lower internal equipment andplacing positioning pins analogous to the pin 22 shown in FIG. 3 in theopenings thus produced.

The core support plate 19 is drilled from above so as to produce all theblind holes 24, each of which is adapted to receive a positioning pin 20of a fuel assembly. Each of the blind holes 24, the axis 23 of which isperfectly aligned in the axial direction of the core of the nuclearreactor, is threaded and opens into a counter bore machined in the uppersurface of the core support plate 5, the diameter of which issubstantially greater than the diameter of the screw threaded blind hole24, so that the screw threaded blind opening 24 is surrounded by a flatannual surface 26 a constituting the base of the counter bore 26.

In the case of the positioning elements fixed underneath the upper coreplate 9, a threaded blind bore is machined from the lower surface of theupper core plate, opening into a counter bore identical to the counterbore 26 shown in FIG. 3, along the lower surface of the upper coreplate.

The positioning pins 22 for the fuel assemblies are all identical,whether they are intended to engage in screw threaded blind openings inthe core support plate 5 or in the upper core plate 9.

The positioning pins 22, the shape of which is shown in FIG. 3, arescrewed inside threaded blind openings in the core support plate 5 andupper core plate 9 until the flange 30 of the positioning pin 22 comesinto contact with the annual flat surface 26 a of the counter bore 26.The pin is tightened against the base of the counter bore with aconstant couple. Then, a cavity 32 in which a gudgeon 30 engages, thefixing of which is completed by spot welds which fix it in the cavity 32in the core support plate or in the upper core plate, is machined in theedge of the flange 30 and in the core support plate or the upper coreplate into which the positioning pin is screwed.

After the nuclear reactor has been powered up, during maintenanceoperations, certain positioning pins for fuel assemblies may deteriorateand have to be replaced in the course of an operation of maintenance andrepair carried out under water in a pool.

The positioning pins 22 are replaced by identical positioning pins whichare put into position after the damaged replacement pins have beenremoved. As will be explained hereinafter, when replacing a positioningpin according to the prior art with a replacement pin according to theinvention, the operations of removing the damaged pins and fixing thereplacement pins are carried out from above the plate to which thedamaged pin is fixed (be it the core support plate or the upper coreplate).

When replacing a damaged pin fixed to the upper surface of the coresupport plate, the operator gains access to the top surface of the coresupport plate after cooling the nuclear reactor, opening the cover ofthe tank, removing the upper internal equipment and the core assemblies,the pool and the reactor tank being filled with water.

When replacing a damaged positioning pin fixed to the upper core plate,at the lower end of the upper internal equipment of the reactor, theremoval of the damaged positioning pin or pins and the installation andfixing of the replacement pins may be carried out after turning theupper internal equipment through 180° on its storage stand, so that thelower surface of the upper core plate is at the upper end of the upperinternal equipment and directed upwards.

In every case, the damaged positioning pins fixed to the core supportplate of the lower internal equipment of the reactor or to the uppercore plate of the upper internal equipment of the reactor are removed bymachining the welded parts of the locking gudgeon 31 of the positioningpin, removing the gudgeon from the cavity 32, then unscrewing thepositioning pin screwed into the blind threaded opening in thecorresponding plate.

In the event that it proves impossible to unscrew the positioning pin,this pin is machined, which may require re-machining and re-boring ofthe blind opening in the corresponding plate. In this case, thedimensions of the threaded portion of the replacement pin should takeaccount of the re-machining and the thread diameter of the blindopening.

An operation to replace a fuel assembly positioning pin producedaccording to the prior art and fixed to a lower core plate 19 of anuclear reactor will now be described.

FIG. 5 shows a positioning element according to the prior art whichcomprises a positioning pin 28 having two main parts, the stud 28 aadapted to project above the core support plate 19 in the axialdirection 23 and a threaded fixing portion 28 b on an axial extension ofthe stud 28 a adapted to be screwed into a blind opening 24 in the coresupport plate 19 threaded over at least part of its length.

The positioning element 20 further comprises a securing washer 29 thecentral opening of which comprises two straight edges, as shown in FIG.6, adapted to cooperate with two flattened areas of a lower part of thepositioning stud 28 a in order to secure the pin 28 against rotationrelative to the securing washer 29.

For putting the positioning element into place on the core support plate19, this operation being carried out in the factory during theconstruction of the nuclear reactor, the washer 29 is engaged on thelower end part of the positioning stud 28 a comprising the flattenedareas and the positioning pin 28 is screwed into the threaded opening 24in the upper core plate 19 until a shoulder of the bushing 28 located atthe lower end of the stud 28 a comes into abutment with the base 26 a ofa circular counter bore 26 machined in the upper part of the coresupport plate 19, in a coaxial arrangement relative to the at leastpartially threaded opening 24. The washer 29 is then secured in thecounter bore 26 by one or more spot welds. In this way the positioningelement 20 is secured in position.

When a fuel assembly is being positioned on the core support plate 19,before the nuclear reactor is put into operation, a foot of the lowerendpiece 15 of the fuel assembly engages the positioning stud 28 a whichcomprises a rounded-off conical upper part so as to ensure that the footof the fuel assembly is engaged and guided and then held by means of acylindrical portion engaging in the opening 21 in the lower endpiece 15with slight play.

Accidentally, one or more studs for positioning fuel assemblies on thecore support plate 19 may be unavailable for positioning a fuel assemblyof the core after a certain period of operation of the nuclear reactor.

Such a defect may be found, for example, where a pin has deteriorated,this deterioration possibly taking the form of a movement of the stud 28a relative to the theoretical axial direction 23, and this deteriorationmay amount to breakage of the pin, for example in the area of smallestdiameter between the stud 28 a and the threaded portion 28 b. In thiscase, the stud 28 a separates from the part of the pin which remainsinside the opening 24 and may become a loose body in the primary circuitof the nuclear reactor. A loose body of this kind is detectedimmediately and requires the equipment of the nuclear reactor to be shutdown, cooled and repaired.

In some case it is also possible that a positioning pin has not beenproperly secured during assembly.

In all these cases, the defective positioning element has to be repairedvery fast, which is done after shut down and cooling of the nuclearreactor, discharge of the core and generally after removal of the lowerinternal equipment from the tank and placing this lower internalequipment on an intervention stand inside the pool of the reactor.

Owing to the need to ensure a perfectly coaxial position between thestud of the positioning element and the opening in the core supportplate 19 and the impossibility of carrying out welding under water onthe irradiated lower internal equipment, a replacement pin 22 as shownin FIG. 3, analogous to a positioning pin of the apparatus according tothe invention is used to carry out the process of restoring thepositioning element 20.

The replacement pin 22 comprises a positioning stud 22 a the shape anddimensions of which are in every point identical to those of the stud 28a of a pin 28 originally fixed to the core support plate 19 of thenuclear reactor. However, the stud 22 a has no flattened areas extendingto its lower end, like the stud 28 a for securing the pin, but only twoflattened areas 27 used as a grouping surface for a screwing tool whenthe replacement pin 22 is screwed in at a final stage of the restorationprocess which will be described hereinafter.

The replacement pin 22 further comprises, on an extension of the axis 23of the pin, a threaded portion 22 b the diameter of which is generallygreater than the diameter of the threaded portion 28 b of the pin 28.Generally, the minimum diameter at the base of the thread of thethreaded portion 28 b of the replacement pin is at least equal to themaximum outer diameter of the threaded portion 28 b of the bushing 28.In this way, in an operation to re-machine the opening 24 in the coresupport plate 19, all the initial threaded portion of this opening whichmay be defective can thus be eliminated.

In an alternative embodiment it is possible to provide a replacementbushing comprising a shaft which is longer than the shaft of a pin 28according to the prior art originally fitted to core support plate 19,and a threaded portion at the end of the shaft of the replacement pinadapted to be screwed into an end portion 24′ of the opening 24 on anaxial extension and below the threaded portion adapted to receive thethreaded portion 28 b of a bushing 28.

Between the stud 22 a and its threaded portion 22 b, the replacementbushing 22 comprises an annular flange 30 and optionally, following theflange 30 in the direction of the threaded portion 28 b, a smoothportion the diameter of which is less than the diameter of the threadedportion 22 b.

The outer diameter of the flange 30 is substantially equal to orslightly less than the diameter of the counter bore 26 machined in theupper part of the core support plate 19 around the opening 24.

The flange 30 comprises a flat lower surface 30 a of annular shapeadapted to come to bear on the surface 26 a of the base of the counterbore, the contact of the surfaces 30 a and 26 a on one another beingintended to ensure perfect positioning of the replacement pin 22 and inparticular the directing of the axis 23 of the positioning stud 22 aperfectly perpendicularly to the upper support surface of the coresupport plate 19. A perfectly coaxial arrangement of the replacement pin22 and the opening 24 is ensured, as will be described hereinafter, bythe precision of construction of the axis of the threaded portion of theopening 24 adapted to receive the threaded portion 22 b of thereplacement bushing 22.

As can be seen from FIGS. 3 and 4, the positioning element 20 mayfurther comprise a gudgeon 31 for securing the replacement pin 22, thegudgeon 31 being tubular and engaging in a cavity 32 machined after thepin 22 has been put into position, through the flange 30 and in thesupport plate 19, the cavity 32 having an axis 32′ substantially at atangent to the outer edge of the flange 30 and the inner edge of thecircular counter bore 26.

FIG. 7 shows a tool support 33 in the shape of a plate for mounting thetools required to carry out the restoration process according to theinvention, on the core support plate 19 of the nuclear reactor.

FIG. 8 shows the support 33 put into position on the upper core plate 19in order to restore a positioning element, at a location 34 close to apartitioning wall 7 of the core.

The tool support 33 consists of a flat metal sheet with raised edges,substantially rectangular in shape and having a rectangular or squarenotch 33 a giving access to position 34 of the positioning element whichis to be restored.

The core support plate 19 shown in FIG. 8 is traversed by wateraccommodation openings 35 for accommodating the water used to cool thecore and the fuel assembly positioning elements each comprising apositioning stud 28 a projecting relative to the support plate 19.

In each of the positions of fuel assemblies of square cross section thecore support plate 19 comprises two positioning elements each having astud 28 a projecting upwards above the core support plate. The elementsfor positioning a fuel assembly are disposed in the angles of the lowerendpiece of the fuel assembly, at the level of the feet of the endpieceresting on the core support plate and arranged along a diagonal of thelower endpiece or the cross section of the fuel assembly. In each of thefuel assembly positions the core support plate 19 is also traversed byfour water accommodation openings 35.

The tool support 33 comprises two openings 36 the diameter of which issubstantially equal to or slightly greater than the diameter of apositioning stud 28 a, the two openings 36 surrounded by a raised edgeof the sheet metal constituting the plate 33 being in arrangementscorresponding to the arrangement of two positioning studs 28 a for afuel assembly adjacent to the assembly in which a positioning element 34is being restored.

The plate 33 for supporting and centering tools is also traversed by anopening 37 at which is placed a gripper 38 the tightening and looseningof which can be remote-controlled by a gripper control motor unit 38 a.The two noses of the grippers 38 are produced so as to define betweenthem a cylindrical opening on an extension of the opening 37 which, in aloosened position of the grippers, engages and introduces a stud 28 a ofa positioning element 28 for a fuel assembly. The opening 37 and thegrippers 38 are arranged relative to the openings 36 and the notch 33 agiving access to the zone of the positioning element 34 so that when theopenings 36 engage on the positioning studs of an assembly adjacent tothe assembly in which a positioning element is being restored, theopening 37 and the grippers 38 engage on a first stud 28 for positioningthe fuel assembly on which the second positioning element 34 is beingrestored level with the notch 33 a in the plate 33.

The tool support 33 further comprises four feet 41 projecting from itslower surface and two handling lugs 39 projecting from its upper surfaceenabling the positioning plate to be transported under water, inside thelower internal equipment of the nuclear reactor, to place it in aservice position as shown in FIG. 8. The tool support 33 is lowered ontothe core support plate 19 so that it is possible to engage the studs 28a of the two positioning elements 28 of the adjacent fuel assembly and,at the same time, the opening 37 and the grippers 38 in the openingposition on the stud 28 a of a first existing positioning element of thefuel assembly in which a missing second positioning element 34 is beingrestored. The tool support plate 33 is lowered until the feet 41 rest onthe top surface of the support plate, the studs 28 a of the positioningelements engaging in the corresponding openings 36 and 37 and betweenthe noses of the grippers 38. The grippers 38 are then actuated so as toachieve precise positioning and placing of the tool support 33.

On its upper surface, the tool support 33 has two positioning studs 42and two fixing bushings 43 operating by screwing a tool which is thuspositioned and fixed precisely on the tool support 33. The position ofthe studs 42 and the screwing and fixing bushings 43 on the tool supportplate 33 is such that the tool engaging on the studs 42 and fixed in thescrewing bushings 43 by corresponding screw-type fixing elements canoperate exactly along the axis of the opening 24 in the positioningelement 34.

FIG. 9 shows the different positioning and fixing elements of thesupport 33 and tools in a service position for restoring a positioningelement 34 for fuel assemblies, adjacent to the partitioning 7.

Before its restoration the positioning element 34 has only the opening24 which does not contain a positioning pin and the internal screwthread of which may have deteriorated and, in the counter bore 26surrounding the opening 24, the washer 29 for securing a pin 28 againstrotation, this pin having previously been removed by machining or lost.

As can be seen from FIG. 10, a tool 44 for machining by electro-erosion,comprising a base 44 a having two openings in positions which enablethem to engage on the studs 42 on the tool support plate 33 and twoscrew-type openings 43 a in positions corresponding to the screwingbushings 43, is mounted on the tool support 33 which has been preciselypositioned on the core support plate 19, so as to ensure on the one handthat the tool support 44 is accurately positioned relative to thepositioning element 34 which is to be restored and on the other handthat the tool 44 has been fixed in place.

The tool 44 is an electro-erosion machine tool comprising, inparticular, a pin 45 at the end of which is fixed an electrode carrier45 a supporting an electrode 45 b (shown in FIG. 11 in particular).

When the tool 44 has been positioned and fixed in place on the support33, the pin 45 carrying the electrode 45 b via the electrode carrier 45a is axially aligned with the opening 24 in the positioning element tobe restored, which comprises only the securing washer 29 lodged in thecounter bore 26 and fixed in position by spot welds 46.

A first operation which should be carried out using the electro-erosiontool 44 is the elimination of the spot welds 46 so that the securingwasher 29 can be removed from the counter bore 26, by an operation whichwill be described hereinafter and which requires the use of anextraction tool mounted on the tool support plate after removal of theelectro-erosion tool 44.

A second operation of machining by electro-erosion inside the opening 24consists in completely removing the original threaded portion of theopening 24 which may be defective and re-machining it to provide a newscrew thread for fitting a replacement pin for the positioning element.

For each of these operations, either a suitably adapted and preadjustedelectro-erosion tool is used comprising an electrode of the desiredshape, or different suitably adapted electrodes, mounted in successionon a standard electro-erosion tool.

A third electro-erosion tool or a third type of electrode is used tocarry out the drilling of the cavity 32 for positioning the tubularsecuring gudgeon 31, as will be described hereinafter. One or moreelectro-erosion machines are used, comprising an electro-erosion headwith three motorised pins X, Y, Z. The direction Z, which corresponds tothe axial direction of the opening 24, is directed along the axis of thepin and of the electrode which is moved in the direction Z to eliminatethe welds 46 from the securing washer 29, from the screw threaded partof the opening 24 and to drill the cavity 32 for accommodating thetubular gudgeon 31.

The movements in directions X and Y are used to carry out the operationswhich require planetary movement of the electrode, for example tomachine the screw thread inside the opening 24. The profile of theelectrode and the travel of the electrode along the axis as well as theworking parameters of the electro-erosion machine make it possible toobtain the desired geometry of the machined surfaces.

The machining operations could also be carried out with conventionalmechanical cutting tools, for example a drill.

However, in every case, the machine chippings have to be collected in acontainer or particles from electro-erosion machining have to berecovered by filtering the water from the pool of the reactor.

As will be explained hereinafter, it is also possible to use guidebarrels for the tools in order to carry out certain machiningoperations. In every case, perfect positioning of the working axis Z ofthe machine tool along the axis of the opening 24 in the positioningelement 34 which is to be restored is achieved by flanging the toolsupport 33 using the grippers 38.

After the welds 46 have been removed from the securing washer 29, theelectro-erosion machine tool is taken away and an extraction tool 47 isfixed to the tool support 33, this extraction tool 47 having a base 47 asimilar to the base 44 a of the electro-erosion tool 44.

The extraction tool 47 comprises in particular grippers 48 a (see FIG.13) comprising four noses and a jack 48 for opening and closing thegrippers 48 a.

The actuating axis for the grippers 48 a is directed along the axis ofthe opening 24 and of the washer 29 which is to be removed from thecounter bore 26, when the tool 47 has been fixed in place on the toolsupport plate 33 mounted so as to restore the positioning element 34.

The tool support 47 also comprises a mechanical connector 47′ allowingactuation of a screw-type jack for moving the grippers 48 a and the jack48 for opening and closing the grippers in the vertical direction,either upwards or downwards.

After the tool 47 has been positioned on the support plate 33, thegripper is brought down into the closed position, i.e. into a positionin which its fingers are close to one another, inside the bore of thewasher 29 which has-remained in place in the counter bore 26.

The jack 48 is then actuated to move the fingers of the grippers apartinside the bore and ensure that the securing washer 29 is gripped. Then,using a pole, the screw-type jack is actuated for raising the grippers48 a and the gripper control jack 48 in the vertical direction Z.

The washer is then recovered or the tool 47 is repositioned, thegrippers 48 a of which have remained in the position of gripping thewasher 29.

Then a machining tool 44 such as an electro-erosion machining tool isput back into position in order to eliminate the original threadedportion from inside the opening 24 in the support plate, this threadedportion possibly being defective.

Other machining means analogous to the electro-erosion machine 44 arethen used to create a new screw thread inside the opening 24, this newscrew thread being produced so as to enable the threaded portion 22 b ofthe replacement bushing 22 described above to be screwed in.

As explained hereinbefore, a replacement bushing 22 can be used,comprising a threaded portion wherein the diameter at the base of thethread is greater than the diameter at the base of the thread of thethreaded portion of the opening 24 which is entirely eliminated, theopening 24 then being machined again at a diameter greater than itsoriginal diameter, as is shown in FIG. 3.

It is also possible to use a replacement bushing having a shaft which islonger than the shaft of a bushing 28 fitted to the core support plateand having a threaded portion at its end. In this case, the screwthreading is provided in the end portion 24′ of the blind opening 24 onan axial extension of the screw thread which has previously beeneliminated.

As shown in FIG. 14, the replacement pin 22 is then screwed into theopening 24 which has been formed and screw threaded. For this, ascrewing tool is used which is operated manually by a pole from abovethe pool of the nuclear reactor and by means of a guiding and grippingtool as shown in FIG. 14. This guiding and gripping tool, generallydesignated 50, comprises a positioning and centering support 50 a whichmay engage on the guide studs 42 and on the screwing bushings 43 of thetool support 33 fixed in place by screws engaging in the screwingbushings 43.

The support 50 a for the guiding and gripping tool comprises a tubularguide 49 integral with the support 50 a, which is placed in a positionperfectly coaxial with the opening 24 when the support 50 a of the tool50 has been put into place on the support plate 33. A screwing tool 51is used comprising a very long pole 51 a, grippers 52 the fingers ofwhich are mounted to be pivotable about axes perpendicular to the axisof the pole 51 a at one end of the pole, and a sleeve 53 for locking andunlocking the grippers which can be controlled to move axially in onedirection or the other, as shown in FIG. 14 (arrow 54), being guided onthe outer surface of the fingers of the grippers 52 and on the end partof the pole 51 a.

From a working position above the level of the pool of the reactor, areplacement bushing 22 is fixed to the end of the screwing tool 51,engaging the gripping ends of the fingers of the grippers 52 on theflattened areas 27 of the grippers 22 and sliding the locking ring 53into its lower locking position. The replacement pin 22 fixed to thegrippers 52 at the end of the pole 51 is lowered into the pool of thereactor until the replacement pin 22 and the grippers 52 held in thelocked position by the sleeve 53 engage in the cylindrical guide 49 ofthe guide tool 50. The end of the replacement pin engages in the entryportion of the opening 24 the axis of which is aligned with the axis ofthe tube guide 49. The replacement pin 22 is screwed into the screwthread which has been remachined in the opening 24 until the flat lowersurface 30 a of the flange 30 comes to bear on the flat base 26 a of thecounterbore 26 machined around the opening 24 in the upper part of thecore support plate 19.

Torque tightening is carried out, to ensure that there is no playbetween the stud and the core support plate. Torque tightening ensuresthat there is no wear that could occur in the presence of play in thereactor in operation.

It should be noted that in the course of the operations to remove thefixing welds from the securing washer 29 and to remove the washer, it isessential to take every precaution to avoid damaging the base 26 a ofthe counterbore 26 which constitutes the abutment surface for thereplacement pin 22 via the flange 30 which determines precisepositioning of the replacement pin 22 and a perfectly verticalpositioning of the positioning stud 22 a.

Finally, the cavity 32 is drilled to accommodate the securing gudgeon 31of the replacement pin.

For this, a machine tool is used, e.g. an electro erosion machine, whichis placed on the tool support plate as described previously. The cavity32 is drilled along the edge of the flange 30 and in an adjacent part ofthe core support plate, the axis 32′ of the drill hole beingsubstantially at a tangent to the edge of the flange 30 and to the edgeof the counterbore 26, as is shown in FIG. 3.

A securing gudgeon 31, which may be a split tubular gudgeon, a drilled1-piece pin or a tubular pin wound with a spiral section is inserted inthe cavity 32. The cavity 32 for receiving the gudgeon 31 may becylindrical or conical with or without a holding chamber and maycomprise a locking groove. The gudgeon engaging in the cavity 32 is heldin place by elasticity. Generally, a tubular gudgeon is used but if asolid cylindrical gudgeon is used it has to be drilled to produce a borefor evacuating water when it is placed in the cavity 32.

As shown in FIG. 6, the gudgeon 31 may be axially secured in the cavity32 machined both in the flange 30 of the replacement pin and in the coresupport plate 19, by slight jagging 19′ of the edge of the cavity 32closest to the core support plate 19 or 30′ of the edge of the cavity 32closest to the flange 30 of the replacement pin 22. This prevents thegudgeon from escaping from its recess 32 during the operation of thenuclear reactor.

Because the replacement pin is generally made of stainless steel 316whereas the core support plate is made of stainless steel 304, thejagging is preferably done on the edge of the cavity 32, closest to theflange 30 of the replacement pin 22 (jagging 30′), owing to the factthat the steel 316 of the replacement pin will withstand work hardeningbetter than the steel 304 of the core support plate 19.

The insertion of the gudgeon 31 in the cavity 32 may be done by impactor by the action of a jack. Jagging is then carried out to ensure thatthe gudgeon cannot be lost.

The drilling of the cavity 32 is carried out in a zone selected as afunction of the position of the holes for the passage of water in thecore support plate and possibly the partitions for the peripheralassemblies of the core, so as to keep the area of the securing gudgeonas far as possible from the edges of the holes for the passage of waterand the partitions.

As shown in FIG. 15 a, it is possible to achieve perfect centring of thetools such as the drilling or tapping tools, directly in the opening 24of the positioning element which is to be restored.

As shown in FIG. 15 a, it is possible to achieve perfect centering ofthe tools such as the drilling or tapping tools, directly in the opening24 of the positioning element which is to be restored.

A positioning tool 54 comprising a centering axis in the base of theopening 24 in the positioning element which is to be restored, below thethreaded portion, is positioned in the opening 24 in the core supportplate, through an opening in the tool support plate 33. The position oforientation of an upper plate of the centering means is fixed, bearingon the support plate 33 through which is provided an opening 55 toenable the drilling zone for receiving the securing gudgeon to bepositioned precisely. The positioning tool 54 is used to position andcentre a guide or barrel for positioning the base of a machine tool.After the removal of the positioning tool, the machining tool is putinto position relative to these centering means. The positioning tool 54could itself be used as a means for centering machining, extraction andscrewing tools.

The devices for positioning fuel assemblies according to the inventionmay be produced and positioned equally well in the upper core plate ofthe lower internal equipment of the reactor as in the upper core plateof the upper internal equipment of the nuclear reactor.

Moreover, these elements can easily be restored when a positioning pinhas been damaged during the handling of fuel assemblies or internalequipment of the nuclear reactor.

Furthermore, in the case of positioning elements produced according tothe prior art comprising a securing washer, the restoration of thesepositioning elements can be carried out by replacing the damagedpositioning pins of the elements according to the prior art withreplacement positioning pins according to the invention.

The invention may apply to any water-cooled nuclear reactor comprisingfuel assemblies resting on a core support plate and held by positioningelements projecting relative to the upper surface of the core supportplate of the nuclear reactor and relative to the lower surface of theupper core plate.

1. In a water-cooled nuclear reactor having: a core that includesjuxtaposed fuel assemblies of straight prismatic shape with theirvertical axes orthogonal to the horizontal core support plate;positioning elements for the fuel assemblies, each element received in arespective opening formed in a lower end piece of the fuel assembly, thepositioning element including— a) a pin having a stud portion receivedwithin a respective opening and projecting from the core support plate,a threaded portion of the pin being screwed into a threaded verticalopening formed in the core support plate; b) a securing washer receivedwithin a counterbore formed around the threaded opening in the coresupport plate and axially mounted on the pin; a restoration process forreplacing a defective positioning element of a fuel assembly, theprocess performed under water and comprising the steps: removing thefuel assemblies from the core; removing the defective positioning pinfrom its threaded opening; machining the threaded opening in the supportplate, under water, to remove the securing washer and to eliminate aoriginal screw thread from the threaded opening; machining a new screwthread where the original screw thread was removed; inserting a modifiedreplacement positioning pin in an associated opening in the lower endpiece of the fuel assembly, a threaded portion of the replacementpositioning pin being screwed into the new screw thread; screwing thereplacement positioning pin until an integral annular flange thereofcontacts the counterbore, the flange having a diameter that issubstantially equal to the diameter of the counterbore; and continuingto screw the replacement positioning pin with a torque which eliminatesplay between the flange and the core support plate during operation ofthe reactor.
 2. The process according to claim 1, together with thesteps: drilling a peripheral zone of the flange of an installedreplacement pin and an adjacent portion of the core support plate toform a cavity; inserting a locking gudgeon into the cavity; andsubjecting the gudeon to elastic deformation within the cavity.
 3. Theprocess according to claim 2, wherein the locking gudgeon is introducedinto the cavity while under water.
 4. The according to claim 2, togetherwith the step of forming a jagged edge at an outer edge of the cavity inorder to retain the gudgeon within the cavity.
 5. The process accordingto claim 4, wherein the formed edge of the cavity is jagged along aperipheral portion of the replacement pin edge.
 6. The process accordingto claim 1, wherein the screw thread of the threaded portion of thereplacement pin is machined so that the minimum diameter of the screwthread of the replacement pin is at least equal to the maximum diameterof the original thread.
 7. The process according to claim 1, wherein thethread of the threaded vertical opening formed in the core support plateis completely removed by machining; and a screw thread, matching athreaded portion of a replacement pin, is axially machined in aninternal blind end portion of the vertical opening formed in the coresupport plate.